The infared optical properties of Sr2RuO4 and SmTiO3 including an object-oriented resistivity interface /

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Abstract

The perovskite crystal structure is host to many different materials from insulating
to superconducting providing a diverse range of intrinsic character and complexity. A
better fundamental description of these materials in terms of their electronic, optical and
magnetic properties undoubtedly precedes an effective realization of their application
potential.
SmTiOa, a distorted perovskite has a strongly localized electronic structure and undergoes
an antiferromagnetic transition at 50 K in its nominally stoichiometric form.
Sr2Ru04 is a layered perovskite superconductor (ie. Tc % 1 K) bearing the same structure
as the high-tem|>erature superconductor La2_xSrrCu04. Polarized reflectance measurements
were carried out on both of these materials revealing several interesting features
in the far-infrared range of the spectrum. In the case of SmTiOa, although insulating,
evidence indicates the presence of a finite background optical conductivity. As the temperature
is lowered through the ordering temperature a resonance feature appears to
narrow and strengthen near 120 cm~^ A nearby phonon mode appears to also couple to
this magnetic transition as revealed by a growing asymmetry in the optica] conductivity.
Experiments on a doped sample with a greater itinerant character and lower Neel temperature
= 40 K also indicate the presence of this strongly temperature dependent mode
even at twice the ordering temperature. Although the mode appears to be sensitive to
the magnetic transition it is unclear whether a magnon assignment is appropriate. At
very least, evidence suggests an interesting interaction between magnetic and electronic
excitations. Although Sr2Ru04 is highly anisotropic it is metallic in three-dimensions at
low temperatures and reveals its coherent transport in an inter-plane Drude-like component
to the highest temperatures measured (ie. 90 K). An extended Drude analysis is
used to probe the frequency dependent scattering character revealing a peak in both the
mass enhancement and scattering rate near 80 cm~* and 100 cm~* respectively. All of
these experimental observations appear relatively consistent with a Fermi-liquid picture
of charge transport.
To supplement the optical measurements a resistivity station was set up with an event
driven object oriented user interface. The program controls a Keithley Current Source,
HP Nano-Voltmeter and Switching Unit as well as a LakeShore Temperature Controller
in order to obtain a plot of the Resistivity as a function of temperature. The system
allows for resistivity measurements ranging from 4 K to 290 K using an external probe
or between 0.4 K to 295 K using a Helium - 3 Cryostat. Several materials of known
resistivity have confirmed the system to be robust and capable of measuring metallic
samples distinguishing features of several fiQ-cm.